Our grandchildren and great-grandchildren may live on a planet even hotter than we feared if countries fail to slash carbon emissions now, according to a new study.

“Our key finding is that if we continue to burn our remaining fossil fuel resources, the Earth will encounter a profound degree of global warming, of 6.4 to 9.5 degrees Celsius [about 11 to 17 degrees Fahrenheit] over 20th-century averages by 2300,” said Katarzyna Tokarska, a climate scientist at the University of Victoria in Victoria, British Columbia, who led the study. The Arctic’s mean temperature could rise about 15 to 20 degrees Celsius (27 to 36 degrees Fahrenheit) over the next century if such trends continue.

Based on current climate trends alone, a recent Princeton University study suggested that up to 200 million people, including many children, could become environmental refugees in the next half-century.

Some earlier studies with relatively simple ocean and land vegetation modeling have suggested that climate change–driven heat increases would flatten out over time despite rising carbon emissions. But Tokarska and her colleagues, using more detailed ocean and plant-cover scenarios, found a linear relationship—that is, as long as the total amount of carbon in the atmosphere keeps rising, so will the temperature.

Burning remaining supplies of coal, oil, and gas would increase total emissions to 5 trillion metric tons over the course of the 2200s, Tokarska said.

“We were looking at the lower bound of the fossil fuel resource estimates, what we know could be burned,” she said. “This could be even higher if we include some of the newer ways of getting at marginal deposits.”

“The results suggest that it would be better to do something now,” she said, “and now is the time to do it.”

The study appears on the heels of record-setting April heat. Scientists from the National Oceanic and Atmospheric Administration announced last week that global temperatures ran 1.43 degrees Fahrenheit above the 20th-century average, the 12th consecutive month temperatures hit a record high.

The Arctic saw even stronger warming, with temperatures in Alaska between January and April averaging more than 11 degrees higher than the historic average.

The Paris climate accord signed last December calls for nations to keep the total global temperature rise to 3.6 degrees Fahrenheit at most, with 2.7 degrees the preferred goal (2 degrees and 1.5 degrees Celsius, respectively). But the commitments nations have made thus far to adopting renewable energy sources and conserving forests are not enough, according to climate scientists.

“This study offers one more piece of evidence that the impacts we can expect from staying on our current path of emissions are more severe than previously thought,” said biologist Juliette Rooney-Varga, director of the Climate Change Initiative at the University of Massachusetts at Lowell. “Even though this study improves upon former models and projections, more likely than not there are biological processes that are not yet incorporated into this kind of modeling.”

Her field research has shown, for instance, that land-based emissions of the potent greenhouse gas methane from the Arctic will likely increase dramatically as the region continues to heat up.

To have a chance of keeping warming at or below 3.6 degrees Fahrenheit, said Rooney-Varga, use of coal, oil, and natural gas must end between 2050 and 2070. “That seems like it might be kind of far off. The reality is it’s actually a very pressing deadline,” she said. “Think of a building built today, expected to remain functional beyond 2065. If it is not built to maximize energy efficiency and use of renewable energy, expensive retrofits or abandonment before the end of its useful life would be needed to meet our climate goals. The same is true for power plants and energy transmission and transportation infrastructure. The faster we act, the less expensive and reckless our transition away from fossil fuels will be.”

“It’s clear we need a mix of energy efficiency [with] solar, wind, hydro, and bioenergy that is carbon neutral,” she added. “The exact mix will depend upon location and resources and how we transmit energy from one place to another.”

Catrina Rorke, director of energy policy at the R Street Institute, a libertarian think tank, said her organization advocates a universal tax on carbon emissions as the best way to encourage low- and no-carbon development, with the money collected used to lower taxes elsewhere. “That puts a direct price on carbon and replaces regulation of greenhouse gases. But we can also strip away things like CAFE standards for automobiles and renewable tax credits,” she said.

Governments would have a role in regulating the free market “to internalize the cost of greenhouse gas emissions,” said Rorke. “I think that the scientific record shows pretty well that CO2 and other greenhouse gases are not priced in the market.”

“I don’t put much stock in international agreements that are predicated on everyone signing a pledge,” she said. “If it doesn’t work for OPEC, why should it work for the climate?”

This story has been revised to reflect the following correction:

Correction: May 24, 2016
An earlier version of this article misstated the rise in temperatures, in Fahrenheit, that this study forecast by 2300. They are about 11 to 17 degrees Fahrenheit in global temperature rise and 27 to 36 degrees Fahrenheit temperature rise in the Arctic.

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